High performance nonlinear optical media

ABSTRACT

In one embodiment this invention provides a high performance nonlinear optical medium which comprises a transparent organic polymer film containing an array of charge asymmetric molecules such as 13,13-diamino-14,14-dicyanodiphenoquinodimethane: ##STR1##

This application is a division of application Ser. No. 089,512 filed8/26/87, now abandoned and a continuation-in-part of patent applicationSer. No. 855,346, filed Apr. 24, 1986, now abandoned which is adivisional filing of patent application Ser. No. 748,617, filed June 25,1985, now U.S. Pat. No. 4,707,303.

BACKGROUND OF THE INVENTION

Nonlinear optics deals with the interaction of light waves due to anelectromagnetic field dependent susceptibility of an opticallytransparent substrate. Nonlinear optical effects are observed at lightintensities which are significant in comparison with the Coulombelectric field binding the electrons in the atoms and molecules of thelight transmitting solid medium. Monochromatic light of the requiredintensity (e.g., 10⁷ v/cm) first became available with the discovery ofthe laser in 1960.

It is known that organic and polymeric materials with large delocalizedπ-electron systems can exhibit nonlinear optical response, which in manycases is a much larger response than that shown by inorganic media.

In addition, the properties of organic and polymeric materials can bevaried to optimize other desirable properties, such as mechanical andthermoxidative stability and high laser damage threshold, withpreservation of the electronic interactions responsible for nonlinearoptical effects.

Thin films of organic or polymeric materials with large second ordernonlinearities in combination with silicon-based electronic circuitryhave potential as systems for laser modulation and deflection,information control in optical circuitry, and the like.

Other novel processes occurring through third order nonlinearity such asdegenerate four-wave mixing, whereby real-time processing of opticalfields occurs, have potential utility in such diverse fields as opticalcommunications and integrated circuit fabrication.

Of particular importance for conjugated organic systems is the fact thatthe origin of the nonlinear effects is the polarization of theπ-electron cloud as opposed to displacement or rearrangement of nuclearcoordinates found in inorganic materials.

Nonlinear optical properties of organic and polymeric division ofPolymer Chemistry at the 18th meeting of the American Chemical Society,September 1982. Papers presented at the meeting are published in ACSSymposium Series 233, American Chemical Society, Washington, D. C. 1983.

The above recited publications are incorporated herein by reference.

There is continuing research effort to develop new nonlinear opticalorganic systems for prospective novel phenomena and devices adapted forlaser frequency conversion, information control in optical circuitry,light valves and optical switches. The potential utility of organicmaterials with large second-order and third-order nonlinearities forvery high frequency application contrasts with the bandwidth limitationsof conventional inorganic electrooptic materials.

Accordingly, it is an object of this invention to provide novel highperformance nonlinear optical media.

It is another object of this invention to provide nonlinear opticalorganic media exhibiting a high χ.sup.(2) susceptibility value.

It is another object of this invention to provide a solid phasenonlinear optical organic medium characterized by a high Miller's delta,an absence of interfering fluorescence, and a high optical damagethreshold.

It is a further object of this invention to provide a nonlinear opticalmedium which comprises a noncentrosymmetric configuration of alignedmolecules having a diphenoquinodimethane conjugated structure.

Other objects and advantages of the present invention shall becomeapparent from the accompanying description and examples.

DESCRIPTION OF THE INVENTION

One or more objects of the present invention are accomplished by theprovision of a nonlinear optical organic medium exhibiting a χ.sup.(2)susceptibility of at least about 1×10⁻⁷ esu.

In another embodiment this invention provides a nonlinear opticalorganic medium exhibiting a χ.sup.(2) susceptibility of at least about1×10⁻⁷ esu, and an absence of interfering fluorescence in the wavelengthrange between about 0.3-3 μm.

In another embodiment this invention provides a nonlinear opticalorganic medium exhibiting a χ.sup.(2) susceptibility of at least about1×10⁻⁷ esu, an absence of interfering fluorescence in the wavelengthrange between about 0.3-3 μm, and an optical loss less than about 1.0decibel per centimeter.

In another embodiment this invention provides a nonlinear opticalorganic medium exhibiting a χ.sup.(2) susceptibility of at least about1×10⁻⁷ esu, an absence of interfering fluorescence in the wavelengthrange between about 0.3-3 μm, an optical loss less than about 1.0decibel per centimeter, and a response time less than about 10⁻¹³second.

In another embodiment the invention provides a nonlinear optical organicmedium exhibiting a χ.sup.(2) susceptibility of at least about 1×10⁻⁷esu, an absence of interfering fluorescence in the wavelength rangebetween about 0.3-3 μm, an optical loss less than about 1.0 decibel percentimeter, a response time less than about 10⁻¹³ second, and phasematching of fundamental and second harmonic frequencies.

In another embodiment the invention provides a nonlinear optical organicmedium exhibiting a χ.sup.(2) susceptibility of at least about 1×10⁻⁷esu, an absence of interfering fluorescence in the wavelength rangebetween about 0.3-3 μm, an optical loss less than about 1.0 decibel percentimeter, a response time less than about 10⁻¹³ second, phase matchingof fundamental and second harmonic frequencies, and a dielectricconstant less than about 5.

In another embodiment this invention provides an optically transparentmedium comprising a noncentrosymmetric or centrosymmetric array ofmolecules having a charge asymmetric diphenoquinodimethane conjugatedstructure.

The term "charge asymmetric" as employed herein refers to the dipolaritycharacteristic of organic molecules containing an electron-withdrawinggroup which is in conjugation with an electron-donating group.

In another embodiment this invention provides a nonlinear opticalorganic medium exhibiting a χ.sup.(2) susceptibility of at least about1×10⁻⁷ esu, an absence of interfering fluorescence in the wavelengthrange between about 0.3-3 μm, an optical loss less than about 1.0decibel per centimeter, a response time less than about 10⁻¹³ second,phase matching of fundamental and second harmonic frequencies, adielectric constant less than about 5, and wherein the medium comprisesa noncentrosymmetric configuration of aligned molecules having a chargeasymmetric diphenoquinodimethane conjugated structure.

In another embodiment this invention provides a nonlinear opticalorganic medium exhibiting a χ.sup.(2) susceptibility of at least about1×10⁻⁷ esu, an absence of interfering fluorescence in the wavelengthrange between about 0.3-3 μm, an optical loss less than about 1.0decibel per centimeter, a response time less than about 10⁻¹³ second,phase matching of fundamental and second harmonic frequencies, adielectric constant less than about 5, and wherein the medium comprisesa noncentrosymmetric configuration of aligned molecules having adiphenoquinodimethane conjugated structure corresponding to the formula:##STR2## where R is a substituent selected from hydrogen and alkylgroups.

In another embodiment this invention provides a nonlinear opticalorganic medium exhibiting a χ.sup.(2) susceptibility of at least about1×10⁻⁷ esu, an absence of interfering fluorescence in the wavelengthrange between about 0.3-3 μm, an optical loss less than about 1.0decibel per centimeter, a response time less than about 10⁻¹³ second,phase matching of fundamental and second harmonic frequencies, adielectric constant less than about 5, and wherein the medium comprisesa noncentrosymmetric configuration of aligned molecules having adiphenoquinodimethane conjugated structure corresponding to the formula:##STR3## where R is a substituent selected from hydrogen and alkylgroups.

The diphenoquinodimethane molecules can have an external field-induceduniaxial molecular orientation in a host liquid medium, or an externalfield-induced stable uniaxial molecular orientation in a host solidmedium.

In another embodiment this invention provides a nonlinear opticalorganic medium exhibiting a Miller's delta of at least about 3 squaremeters/coulomb.

In another embodiment this invention provides a solid phase nonlinearoptical organic medium characterized by a Miller's delta of at leastabout 3 square meters/coulomb, a vapor pressure less than about 10⁻⁶torr, and an optical damage threshold of at least about one gigawatt persquare centimeter.

This invention further contemplates the provision of an opticallytransparent medium comprising a noncentrosymmetric or centrosymmetricconfiguration of a 13,13-diamino-14,14-dicyanodiphenoquinodimethane typeor a 13,13-diamino-14,14-dicyano-4,5,9,10-tetrahydropyrenoquinodimethanetype of molecules, wherein the amino groups can be either substituted orunsubstituted.

In a further embodiment this invention provides a nonlinear opticalmedium comprising a solid polymeric medium having incorporated therein adistribution of 13,13-diamino-14,14-dicyanodiphenoquinodimethane or13,13-diamino-14,14-dicyano-4,5,9,10-tetrahydropyrenoquinodimethanemolecules, wherein the amino groups can be either substituted orunsubstituted.

The term "Miller's delta" as employed herein with respect to secondharmonic generation (SHG) is defined by Garito et al in Chapter 1,"Molecular Optics:Nonlinear Optical Properties Of Organic And PolymericCrystals"; ACS Symposium Series 233 (1983).

The quantity "delta"(δ) is defined by the equation:

    d.sub.ijk =ε.sub.o χ.sub.ii.sup.(1) χ.sub.jj.sup.(1) χ.sub.kk.sup.(1) δ.sub.ijk

where terms such as χ_(ii).sup.(1) are the linear susceptibilitycomponents, and d_(ijk), the second harmonic coefficient, is definedthrough

    χ.sub.ijk.sup.(2) (-2ω; ω,ω)=2d.sub.ijk (-2ω; ωω)

The Miller's delta (10⁻² m² /c at 1.06 μm) of various nonlinear opticalcrystalline media are illustrated by KDP (3.5), LiNbO₃ (7.5), GaAs(1.8)and 2-methyl-4-nitroaniline (160).

Such comparative figures of merit are defined over the frequency rangeextending to zero frequency, or equivalently DC, The term "fluorescence"as employed herein refers to an optical effect in which a molecule isexcited by short wavelength light and emits light radiation at a longerwavelength. The fluorescence effect is described with respect to liquiddye lasers in "Optoelectronics, An Introduction", pages 233-236,Prentice Hall International, Englewood Cliffs, New Jersey(1983).

The term "optical loss" as employed herein is defined by the equation:

    αL=10 log (I.sub.o /I)

where

α=attenuation coefficient ratio of lost light per unit length

L=sample length

I_(o) =intensity of incident light

I=intensity of transmitted light.

The term "optical scattering loss" is defined and measuredquantitatively by ##EQU1## where T.sub.⊥ is the transmission of opticalradiation through the test sample between polarizers perpendicular toeach other, and T.sub.|| is the transmission between polarizers parallelto each other.

The term "response time" as employed herein refers to numerous physicalmechanisms for nonlinear optical responses and properties of nonlinearoptical materials. The fastest intrinsic response time to lightradiation is a physical mechanism based on electronic excitationscharacterized by a response time of about 10⁻¹⁴ -10⁻¹⁵ seconds. Responsetime is a term descriptive of the time associated with optical radiationcausing promotion of an electron from the electronic ground state to anelectronic excited state and subsequent de-excitation to the electronicground state upon removal of the optical radiation.

The term "phase matching" as employed herein refers to an effect in anonlinear optical medium in which a harmonic wave is propagated with thesame effective refractive index as the incident fundamental light wave.Efficient second harmonic generation requires a nonlinear optical mediumto possess propagation directions in which optical medium birefringencecancels the natural dispersion, i.e., the optical transmission offundamental and second harmonic frequencies is phase matched percentageof the incident light to the second harmonic wave.

For the general case of parametric wave mixing, the phase matchingcondition is expressed by the relationship:

    n.sub.1 ω.sub.1 +n.sub.2 ω.sub.2 =n.sub.3 ω.sub.3

where n₁ and n₂ are the indexes of refraction for the incidentfundamental radiation, n₃ is the index of refraction for the createdradiation, ω₁ and ω₂ are the frequencies of the incident fundamentalradiation and ω₃ is the frequency of the created radiation. Moreparticularly, for second harmonic generation, wherein ω₁ and ω₂ are thesame frequency ω, and ω₃ is the created second harmonic frequency, 2ω,the phase matching condition is expressed by the relationship:

    n.sub.ω =n.sub.2ω

where n.sub.ω and n₂ω are indexes of refraction for the incidentfundamental and created second harmonic light waves, respectively. Moredetailed theoretical aspects are described in "Quantum Electronics" byA. Yariv, chapters 16-17 (Wiley and Sons, New York, 1975).

The term "dielectric constant" as employed herein is defined in terms ofcapacitance by the equation: ##EQU2## where

C=capacitance when filled with a material of dielectric constant ε

C_(o) =capacitance of the same electrical condenser filled with air.

The term "external field" as employed herein refers to an electric, ormagnetic or mechanical stress field which is applied to a medium ofmobile organic molecules, to induce dipolar alignment of the moleculesparallel to the field.

The term "optically transparent" as employed herein refers to an opticalmedium which is transparent or light transmitting with respect toincident fundamental light frequencies and created light frequencies. Ina nonlinear optical device, a present invention nonlinear optical mediumis transparent to both the incident and exit light frequencies.

Nonlinear Optical Properties

The fundamental concepts of nonlinear optics and their relationship tochemical structures can be expressed in terms of dipolar approximationwith respect to the polarization induced in an atom or molecule by anexternal field.

As summarized in the ACS Symposium Series 233(1983), the fundamentalequation (1) below describes the change in dipole moment between theground state μ_(g) and an excited state μ_(e) expressed as a powerseries of the electric field E which occurs upon interaction of such afield, as in the electric component of electromagnetic radiation, with asingle molecule The coefficient α is the familiar linear polarizability,β and γ are the quadratic and cubic hyperpolarizabilities, respectively.The coefficients for these hyperpolarizabilities are tensor quantitiesand therefore highly symmetry dependent. Odd order coefficients arenonvanishing for all structures on the molecular and unit cell level.The even order coefficients such as β are non-zero for those structureslacking a center of inversion symmetry on the molecular and unit celllevel.

Equation (2) is identical with (1) except that it describes amacroscopic polarization, such as that arising from an array ofmolecules in a crystal.

    μΔ=μ.sub.e -μ.sub.g =αE+βEE+γEEE+(1)

    P=P.sub.0 +χ.sup.(1) E+χ.sup.(2) EE+χ.sup.(3) EEE+(2)

Light waves passing through an array of molecules can interact with themto produce new waves. This interaction may be interpreted as resultingfrom a modulation in refractive index or alternatively as a nonlinearityof the polarization. Such interaction occurs most efficiently whencertain phase matching conditions are met, requiring identicalpropagation speeds of the fundamental wave and the harmonic wave.Birefringent crystals often possess propagation directions in which therefractive index for the fundamental ω and the second harmonic 2ω areidentical so that dispersion may be overcome.

A present invention organic medium typically is optically transparentand exhibits hyperpolarization tensor properties such as second harmonicand third harmonic generation, and the linear electrooptic (Pockels)effect. For second harmonic generation, the bulk phase of the organicmedium whether liquid or solid does not possess a real or orientationalaverage inversion center The medium is a macroscopic noncentrosymmetricstructure.

Harmonic generation measurements relative to quartz can be performed toestablish the value of second-order and third order nonlinearsusceptibility of the optically clear media.

In the case of macroscopic nonlinear optical media that are composed ofnoncentrosymmetric sites on the molecular and unit cell level, themacroscopic second order nonlinear optical response χ.sup.(2) iscomprised of the corresponding molecular nonlinear optical response B Inthe rigid lattice gas approximation, the macroscopic susceptibilityχ.sup.(2) is expressed by the following relationship:

    χ.sub.ijk (-ω.sub.3 ; ω.sub.1,ω.sub.2)=NF.sup.ω.sbsp.3 f.sup.ω.sbsp.2 f.sup.ω.sbsp.1 <β.sub.ijk(-ω.sub.3 ; ω.sub.1,ω.sub.2)>

wherein N is the number of sites per unit volume, f represent smalllocal field correlations, β_(ijk) is averaged over the unit cell, ω₃ isthe frequency of the created optical wave, and ω₁ and ω₂ are thefrequencies of the incident fundamental optical waves.

These theoretical considerations are elaborated by Garito et al inchapter 1 of the ACS Symposium Series 233 (1983) recited hereinabove;and by Lipscomb et al in J. Chem. Phys., 75, 1509 (1981), incorporatedby reference. See also Lalama et al, Phys. Rev., A20, 1179 (1979); andGarito et al, Mol. Cryst. and Liq. Cryst., 106, 219 (1984); incorporatedby reference.

Field-induced Microscopic Nonlinearity

The microscopic response, or electronic susceptibility β, and itsfrequency dependence or dispersion, is experimentally determined byelectric field induced second harmonic generation (DCSHG) measurementsof liquid solutions or gases as described in "Dispersion Of TheNonlinear Second Order Optical Susceptibility Of Organic Systems",Physical Review B, 28 (No. 12), 6766 (1983) by Garito et al, and theMolecular Crystals and Liquid Crystals publication cited above.

In the measurements, the created frequency ω₃ is the second-harmonicfrequency designated by 2ω, and the fundamental frequencies ω₁ and ω₂are the same frequency designated by ω. An applied DC field removes thenatural center of inversion symmetry of the solution, and the secondharmonic signal is measured using the wedge Maker fringe method. Themeasured polarization at the second harmonic frequency 2ω yields theeffective second harmonic susceptibility of the liquid solution and thusthe microscopic susceptibility β for the molecule.

For purposes of the present invention, a class of organic compoundswhich exhibit extremely large values of β is one containing anoncentrosymmetric diphenoquinodimethane structure. Illustrative of thisclass of compounds is 13,13-diamino-14,14-dicyanodiphenoquinodimethane(DCNDQA): ##STR4##

The DCNDQA molecule is characterized by a single excited state at 2.2eV(0.6 ); a dipole moment difference of Δμ₁ ^(x) :23D; a transitionmoment of μ^(x) _(1g) :13.6D; and large 2ω and ω contributions to β oforder 10³ at 1μ-0.6μ, and no interfering 2ω resonance from higherexcitations.

Solid Organic Guest-host Media

In one of its embodiments this invention provides nonlinear opticallytransparent polymeric media having incorporated therein a distributionof dipolar diphenoquinodimethane guest molecules.

Illustrative of this type of optical medium is a methyl methacrylatefilm containing a distribution of DCNDQA molecules.

If the distribution of guest molecules is random, there is orientationalaveraging by statistical alignment of the dipolar molecules in thepolymeric host, and the optical medium exhibits third order nonlinearity(χ.sup.(3)).

If the distribution of guest molecules is at least partially uniaxial inmolecular orientation, then the optical medium exhibits second ordernonlinearity (χ.sup.(2)). One method for preparing polymeric films withlarge second-order nonlinear coefficients is to remove the orientationalaveraging of a dopant molecule with large β by application of anexternal DC electric field or magnetic field to a softened film. Thiscan be accomplished by heating the film above the host polymerglass-transition temperature T_(g), then cooling the film below T_(g) inthe presence of the external field The poling provides the alignmentpredicted by the Boltzmann distribution law.

The formation of a thin host polymer medium containing guest moleculeshaving, for example, uniaxial orthogonal molecular orientation can beachieved by inducing a dipolar alignment of the guest molecules in themedium with an externally applied field of the type described above.

In one method a thin film of the polymer (e.g., methyl methacrylate)containing guest molecules (e.g., DCNDQA) is cast between electrodeplates. The polymer medium then is heated to a temperature above thesecond order transition temperature of the polymer. A DC electric fieldis applied (e.g., at a field strength between about 400-100,000 V/cm)for a period sufficient to align all of the guest molecules in aunidirectional orthogonal configuration parallel to the transversefield. Typically the orientation period will be in the range betweenabout one second and one hour, as determined by factors such as guestmolecular structure and field strength.

When the orientation of guest molecules is complete, the polymer mediumis cooled below its second order transition temperature, while themedium is still under the influence of the applied DC electric field Inthis manner the uniaxial orthogonal molecular orientation of guestmolecules is immobilized in a rigid structure.

The uniaxial molecular orientation of the guest molecules in the polymermedium can be confirmed by X-ray diffraction analysis. Another method ofmolecular orientation measurement is by optical characterization, suchas optical absorption measurements by means of a spectrophotometer witha linear polarization fixture.

Quinodimethane Compounds

Another aspect of the present invention is the utilization of aquinodimethane compound as a charge asymmetric component of nonlinearoptical media.

The quinodimethane structures of particular interest are thosecorresponding to the formulae: ##STR5## where R is hydrogen or an alkylgroup. Illustrative of alkyl groups are methyl, ethyl, propyl,isopropyl, butyl, isobutyl, pentyl, decyl, hexadecyl, eicosyl, and thelike. Alkyl groups containing between about 1-20 carbon atoms arepreferred. The NR₂ group can also represent a heterocyclic group such aspiperidyl, piperizyl or morpholinyl.

The ═C(NR₂)₂ moiety in the formulae can constitute a heterocyclicradical in which the two amino groups taken together with the connectingmethylidene carbon atom form a cyclic structure such as imidazole in thequinodimethane compounds: ##STR6##

The quinodimethane compounds can also contain substituents which haveone or more optically active asymmetric centers, such as chiral isomericstructures corresponding to the formulae: ##STR7##

In the above-illustrated structural formulae, the cyclic groups can haveone or more of the hydrogen positions on the ring carbon atoms replacedwith a substituent such as alkyl, halo, alkoxy, phenyl, and the like, orcan be integrated as part of a more complex fused polycyclic ringstructure.

The quinodimethane compounds are more fully described in U.S. Pat. No.4,640,800; and copending patent application Ser. No. 8764,203, filed May19, 1986.

The following examples are further illustrative of the presentinvention. The components and specific ingredients are presented asbeing typical, and various modifications can be derived in view of theforegoing disclosure within the scope of the invention.

Fluorescence activity in a nonlinear optical medium is measured byPerkin-Elmer Fluorescence Spectroscopy Model No. MPF-66 or LS-5.

Optical loss exhibited by a nonlinear optical medium is measured byoptical time domain reflectometry or optical frequency-domainreflectometry as described in "Single-mode Fiber Optics" by Luc B.Jeunhomme, Marcel Dekker Inc., N.Y., 1984. It is also measured by themethod described in "The Optical Industry And Systems PurchasingDirectory", Photonics, 1984. The optical scattering loss isquantitatively measured by the ratio of perpendicular transmission toparallel transmission of a He-Ne laser beam through the nonlinear samplewhich is placed between crossed polarizers.

The response time of a nonlinear optical medium is calculated by themethod described in "Optoelectronics; An Introduction" by P. J. Deau,Editor, Prentice-Hall International.

The dielectric constant of a nonlinear optical medium is measured by themethods described in Chapter XXXVIII of "Technique of OrganicChemistry", Volume I, Part III, (Physical Methods of Organic Chemistry)by Arnold Weissberger, Editor, Interscience Publishers Ltd., New York,1960.

EXAMPLE I

This Example illustrates the preparation of13,13-diamino-14,14-dicyano-4,5,9,10-tetrapyrenoquinodimethane inaccordance with the present invention.

Ten grams of13,13,14,14-tetracyano-4,5,9,10-tetrahydropyrenoquinodimethane preparedby the synthetic scheme previously described and 2 liters oftetrahydrofuran are placed in a three-necked three-liter flask equippedwith a mechanical stirrer, a nitrogen inlet, a drying tube and agas-inlet connected to an anhydrous ammonia gas tank. Ammonia gas isbubbled through the stirred solution for three days at room temperature.The crude product in precipitate form is filtered from the reactionmixture, washed with distilled water, and recrystalized from DMF-waterto yield high purity13,13-diamino-14,14-dicyano-4,5,9,10-tetrahydropyrenoquinodimethaneproduct. DC induced second harmonic generation can achieve a secondorder nonlinear optical susceptibility χ.sup.(2) of about 1.5×10⁻⁷ esu,and a Miller's delta of about 4 square meters/coulomb in the product.

When a NLO medium of the product is centrosymmetric in macroscopicconfiguration, it can exhibit a nonlinear optical susceptibilityχ.sup.(3) of about 2.5×10⁻⁹ esu, a response time of less than 10⁻¹³second, an absence of fluorescence in the wavelength range between about0.3-3 μm, an optical loss less than about 1.0 decibel per centimeter,and a dielectric constant less than about 5.

EXAMPLE II

This Example illustrates the preparation of13,13-di(n-hexydecylamino)-14,14-dicyano-4,5,9,10-tetrahydropyrenoquinodimethanein accordance with the present invention.

A three-necked three-liter flask equipped with a mechanical stirrer, anitrogen inlet, a drying tube, and an addition funnel is charged with 10grams (0.03 moles) of13,13,14,14-tetracyano-4,5,9,10-tetrahydropyrenoquinodimethane and twoliters of tetrahydrofuran Twenty-nine grams (0.12 moles) ofn-hexadecylamine in 100 ml of tetrahydrofuran is added dropwise into theflask, and the resulting mixture is stirred for three days at roomtemperature The resulting THF solution is concentrated on a rotaryevaporator.

The crude product in precipitate form is separated by filtration, washedwith distilled water, neutralized with 10% solution of ammoniumhydroxide, washed with water, and then recrystallized from N,N-dimethylformamide-water to yield13,13-di(n-hexadecylamino)-14,14-dicyano-4,5,9,10-tetrahydropyrenoquinodimethane.This compound is aligned in a melt-phase in a DC field by applying about15K volts/cm, and cooled slowly to freeze the aligned molecularstructure in the DC field. The aligned molecular medium is opticallytransparent and can exhibit a second order nonlinear opticalsusceptibility χ.sup.(2) of about 1.5×10⁻⁷ esu, and a Miller's delta ofabout 4 square meters/coulomb.

In a medium in which the molecules are randomly distributed, the productcan exhibit a third order nonlinear optical susceptibility χ.sup.(3) ofabout 2.5×10⁻⁹ esu. The other properties are similar to those describedfor the Example I product.

EXAMPLE III

This Example illustrates the preparation of13,13-diamino-14,14-dicyanodiphenoquinodimethane in accordance with thepresent invention.

Following the procedure of Example I,13,13-diamino-14,14-dicyanodiphenoquinodimethane is prepared by ammoniatreating a tetrahydrofuran solution containing 10 grams of13,13,14,14-tetracyanodiphenoquinodimethane that is obtained by thesynthesis scheme previously described.

DC induced second harmonic generation can provide a second ordernonlinear optical susceptibility χ.sup.(2) of about 1×10⁻⁷ esu in theproduct.

In a product medium with a centrosymmetric molecular configuration, thesusceptibility χ.sup.(3) can be about 2×10⁻⁹ esu. The other mediumproperties are similar to those described for the Example I product.

EXAMPLE IV

This Example illustrates the preparation of13,13-di(n-hexyldecylamino)-14,14-dicyanodiphenoquinodimethane.

Following the procedure of Example II,13,13-di(n-hexadecylamino)-14,14-dicyanodiphenoquinodimethane isprepared by employing a tetrahydrofuran solution containing ten grams of13,13,14,14-tetracyanodiphenoquinodimethane and thirty-two grams ofn-hexadecylamine The second order nonlinear optical susceptibilityχ.sup.(3) can be about 1×10⁻⁷ esu after alignment of molecules in a DCfield, or after alignment of molecules by the Langmuir-BlodgettTechnique in which a monolayer or several layers of molecules aredeposited on a glass substrate.

EXAMPLE V

This Example illustrates the use of13,13-di(n-hexyldecylamino)-14,14-dicyano-4,5,9,10-tetrahydropyrenoquinodimethaneas a guest molecule in a polymer medium.

Ten grams of13,13-di(n-hexadecylamino)-14,14-dicyano-4,5,9,10-tetrahydropyrenoquinodimethaneand 90 grams of poly(methyl methacrylate) are dissolved in 400 ml ofmethylene chloride. A film (2 mil) is cast from this solution on a glassplate coated with indium tin oxide Another glass plate coated withindium tin oxide is placed on the film, and then the film is heated toabout 150° C. A DC field is applied to align the molecules, and the filmis cooled slowly in the applied field to yield an aligned polymer alloywhich can have a second order nonlinear susceptibility χ.sup.(2) ofabout 1×10⁻⁷ esu.

What is claimed is:
 1. A nonlinear optical organic medium exhibiting aχ.sup.(2) susceptibility of at least about 1×10⁻⁷ esu, an absence ofinterfering fluorescence in the wavelength range between about 0.3-3 μm,an optical loss less than about 1.0 decibel per centimeter, a responsetime less than about 10⁻¹³ second, phase matching of fundamental andsecond harmonic frequencies, and a dielectric constant less than about5; and wherein the medium comprises a noncentrosymmetric configurationof aligned molecules having a diphenoquinodimethane ornaphthoquinodimethane conjugated structure.
 2. A nonlinear opticalorganic medium in accordance with claim 1 wherein the aligned moleculescorrespond to the structure: ##STR8## where R is a substituent selectedfrom hydrogen and alkyl groups.
 3. A nonlinear optical organic medium inaccordance with claim 1 wherein the aligned molecules correspond to thestructure: ##STR9## where R is a substituent selected from hydrogen andalkyl groups.